This invention relates to the supplying of material in particulate form from one of several sources to one of several destinations by pneumatic conveying. More particularly, this invention relates to a system and method for supplying plastic in particulate form from several material dryers to several individual processing stations having processing equipment, such as injection molding machinery. This invention also relates to such a system and method where either the source or the destination may be changed easily.
In the manufacture of plastic articles, it is often desirable to produce articles of the same shape but using different plastic materials or different colors of the same plastic material. One way in which to achieve this result is to have several identical molds, each of which is attached to a holding area for a different type of plastic. Then each type of plastic may be delivered to the particulate mold when desired. However, this configuration does not entail use of the molds to their full capacity, resulting in unnecessary expense among other problems. Alternatively, there may be provided several sources of material which may feed a single injection molding machine. However, in this configuration, a shutdown of a single injection molding machine means that several sources of supply are not in use, which is wasteful of both time and money.
One solution to these problems would be the provision of a number of sources that can reach a number of processing stations. However, in various applications, the presence of hygroscopic materials in lines leading from sources to destinations requires that all material be cleared from the lines. Many plastics in particulate form are hygroscopic. Such materials, when not in a temperature and humidity controlled environment, can absorb water from the air. If such materials remain in the line or tube used for conveying the material water will be absorbed. Moreover, material that is being maintained in a hopper at a processing station is generally in an uncontrolled environment. The longer that material remains in such an uncontrolled environment at a processing station, the more moisture is absorbed. If materials that have absorbed water are included in the material used in injecting molding, the resulting molded materials will be of less than acceptable quality.
Changing materials presents additional problems. There is necessarily at least one section of tubing through which different types of plastic material will pass. In these sections, residual material may be different from material newly entering the tube. Thus, impurities will be found in the material used in injection molding. As a result, articles of less than acceptable quality will be produced.
Disassembly and cleaning of the system would eliminate the problems associated with residual material. However, such disassembly and cleaning are expensive and time-consuming, since shutdown of the material delivery system is required. Such shutdowns would increase the length of time between deliveries of material to processing stations. In order to avoid shutdowns at processing stations to wait for material as a result of the increased length of time between deliveries, it is desirable to increase the amount of material stored at the processing station. Increasing the quantities of material stored at the processing station will increase the amount of time such material is in an uncontrolled environment resulting in absorption of water and poor product quality.